Effects of understory characteristics on browsing patterns of roe deer in central European mountain forests

Abstract Selective browsing by deer on young trees may impede the management goal of increasing forest resilience against climate change and other disturbances. Deer population density is often considered the main driver of browsing impacts on young trees, however, a range of other variables such as food availability also affect this relationship. In this study, we use browsing survey data from 135 research plots to explore patterns of roe deer (Capreolus capreolus) browsing pressure on woody plants in mountainous forests in central Europe. We fitted species‐specific generalised linear mixed models for eight woody taxa, assessing the potential effects of understory characteristics, roe deer abundance and lying deadwood on browsing intensity. Our study reveals conspecific and associational effects for woody taxa that are intermediately browsed by roe deer. Selective browsing pressure was mediated by preferences of plants, in that, browsing of strongly preferred woody taxa as for example mountain ash (Sorbus aucuparia) and of least preferred woody taxa, for example Norway spruce (Picea abies) was not affected by the surrounding understory vegetation, while browsing pressure on intermediately browsed species like for example silver fir (Abies alba) was affected by understory characteristics. Contrary to our expectations, roe deer abundance was only positively associated with browsing pressure on silver fir and bilberry (Vaccinium myrtillus), while all other plants were unaffected by deer abundance. Finally, we did not find an influence of lying deadwood volume on the browsing pressure on any woody‐plant species. Overall, our results indicate that patterns in browsing preference and intensity are species‐specific processes and are partly affected by the surrounding understory vegetation. Current management strategies that aim to reduce browsing pressure through culling may be inefficient as they do not address other drivers of browsing pressure. However, managers also need to consider the characteristics of the local understory vegetation in addition to deer abundance and design species‐specific plans to reduce browsing on woody plant taxa.


| INTRODUC TI ON
Browsing on young trees by ungulates can be problematic for forest managers who are motivated to increase the resilience of forests towards climate change. In central Europe, large amounts of Norway spruce (Picea abies (L.) H.Karst.) stands have been destroyed by bark beetle infestations in the last decade (Biedermann et al., 2019).
While bark beetle calamities are natural disturbances, the frequency of infestation events is increasing due to climate change (Bentz et al., 2010;Cudmore et al., 2010). One way to make forests more resilient to disturbances, such as climate change and frequent pest infestations, is to increase tree species richness (Berthelot et al., 2021;Jactel et al., 2017). In mountainous regions of central Europe, this is usually achieved by increasing the proportions of silver fir (Abies alba MILL.) and deciduous tree species in the canopy (Lebourgeois et al., 2013;Schwarz & Bauhus, 2019). However, efforts to improve heterogeneity in forests are undermined by growing deer abundances, which increase the extent to which young trees are browsed (Häsler & Senn, 2012;Moser et al., 2006;Senn & Suter, 2003). Deer browsing reduces the survival of young trees (Ammer, 1996), delays regeneration (Kupferschmid et al., 2019), and can reduce tree species diversity through selective browsing (Perea et al., 2014;Ward & Williams, 2020). Consequently, browsing by large herbivores might inhibit the development of forest resilience (e.g. against climate change) by affecting long-term tree species composition (Champagne et al., 2021;Meier et al., 2017).
Roe deer (Capreolus capreolus L.) are the most common and widespread ungulates in central Europe (Lorenzini et al., 2022). As concentrate selectors Tixier & Duncan, 1996), roe deer are highly adaptable in their feeding behaviour König et al., 2020), and consume a variety of vegetation types. The proportion of woody plant consumption in roe deer diet depends on the availability of other forage in their environment. For example, woody plants make up a greater proportion of roe deer diet in the winter compared to the summer because there are fewer nonwoody plants available in winter Häsler & Senn, 2012;Tixier & Duncan, 1996). As roe deer populations become more abundant, the overall browsing pressure in forest systems increases Hothorn & Müller, 2010;Tremblay et al., 2007). However, where and how intensely the browsing occurs is influenced by factors such as forest structure (Kupferschmid et al., 2020), predation pressure (Kuijper et al., 2013), anthropogenic disturbance Gerhardt et al., 2013;Möst et al., 2015) and surrounding land-use (Takarabe & Iijima, 2020).
The availability of forage influences the browsing intensity on individual plants through associational effects (Hagen & Suchant, 2020;Moser et al., 2006;Skoták et al., 2021;Underwood et al., 2014;Ward et al., 2008). In the case of 'associational susceptibility', the availability of other potential-forage plants increases the overall attraction of a vegetation patch, thus intensifying the browsing pressure on individual plants (Milligan & Koricheva, 2013;Vehviläinen et al., 2007). Where individual young trees are browsed on less because the browsing pressure is deflected to other available forage, the composition of the understory reduces browsing damage on woody vegetation, demonstrating 'associational resistance' (Champagne, Moore, et al., 2018;Milligan & Koricheva, 2013). The probability of an individual woody plant being browsed is also affected by proximity and abundance of conspecifics ('conspecific effects'). Thus, the abundance of forage may motivate habitat/ patch selection, while the diversity of the understory might affect browsing pressure on individual plants (Häsler & Senn, 2012), and the likelihood of an individual plant being browsed is affected by the presence of conspecifics (Champagne et al., 2020;Otway et al., 2005;Underwood et al., 2014). Browsing intensity has been previously described in relation to the diversity of woody vegetation (Ameztegui & Coll, 2015;Ohse et al., 2017;Ward et al., 2008), however, the role of associational effects and conspecific effects on the intensity by which woody plants are browsed understory is understudied.
Browsing damage is typically mitigated through culling of herbivores because abundance and access of individuals to forage is thought to be the main driver of browsing intensity (Hothorn & Müller, 2010). In some cases, reducing herbivore density and abundance is a successful intervention (Jenkins et al., 2014), however, often, hunting alone does not reduce browsing damage sufficiently (Kamler et al., 2010;Wright et al., 2012). Counterproductively, high and continuous hunting pressure can even increase damage by deer locally because deer use local resources more intensively to avoid risks through movement (Gerhardt et al., 2013;Nopp-Mayr et al., 2011). The relationship between herbivore abundance and browsing pressure is not yet fully understood and requires further investigation.
Retention forestry has emerged as a practice to conserve forest biodiversity within production forests, by retaining old-growth features like deadwood and old trees (Gustafsson et al., 2020). Next to contributing to the conservation of saproxylic species, the retention of lying deadwood may indirectly facilitate the natural succession of young trees through physical protection from herbivore access (Pellerin et al., 2010;Whyte & Lusk, 2019). While the effectiveness of retaining deadwood on biodiversity conservation has been explored, less is known about the effect of deadwood on mitigating browsing pressure on woody vegetation Hall Defrees et al., 2021;Pellerin et al., 2010;van Ginkel et al., 2021).
In this study, we aimed to assess the potential associational effects of understory vegetation on the browsing impact of roe

| Browsing survey
We conducted browsing surveys on all 135 research plots in autumn (October-November) 2019 (n = 71) or 2020 (n = 64) to assess summer browsing on woody plants. In the following spring (March-April), we repeated the surveys to assess browsing intensity over the full year. We assessed all woody vegetation between 8 and 130 cm of height, including young trees, shrubs, and dwarf-shrubs, within 1 m of each side of three 15 m long transects ( Figure 1).
Assessment included recording the species, height, and position in relation to conspecifics (Grouping, Table 1). We counted the number of trees from each species on the transect, and for each tree species, we surveyed up to 20 individuals for signs of browsing.
For shrubs in clonal colonies, each ramet was treated as an individual, as true separation of ramets was impossible to determine without removing the plant from the soil. As such, for Rubus spp.
and Vaccinium myrtillus (L.), percent of coverage was estimated instead of counting individuals. If more than 20 individuals were found in a transect, we aimed to sample a set of individuals spread over the entire length of the transect as well as along the entire range of height and grouping constellations. We assigned each individual an index of browsing intensity, which was the proportion of browsed branches of the 10 highest branches. If the plant had less than 10 branches, all branches were assessed. Browsing transects were positioned at fixed points in the northwest corner, the centre, and the southeast corner of the plots (Figure 1).
Transects ran from the southwest to northeast, unless plots had a distinct slope, in which case transects were always parallel to the F I G U R E 1 Schematic of methodological set-up of 2 × 15 m 2 browsing transects (black bars) and camera trap positions on onehectare research plots.
slope. To account for overall forage availability, we estimated the percentage of cover of understory vegetation (UnderC), including young trees, dwarf shrubs, herbs, grasses, sedges, and ferns within 5 m of the transects. Other vegetation-related variables assessed were: the total number of young trees in the transect (TotalWP), the total number of conspecifics in the transect and the plot-level diversity of the understory (UnderDiv, Table 1).

| Camera trapping
To assess the relative abundance of roe deer (RDeer), we used detection rates of camera traps, collected in five camera trapping periods, during spring in 2019-2021, and autumn in 2019-2020 (Rovero & Marshall, 2009). In every camera trapping period, we placed one camera trap close to one of the three transects ( Figure 1). We assigned the first position randomly and henceforth systematically shifted within these three positions through the five seasons. For a detailed description of the camera trap setup see Schwegmann et al. (2023). We summed counts of roe deer events per season and corrected for trapping effort. Roe deer events were deemed independent when detections were at least 5 minutes apart. For this study, the mean number of events per trapnight, averaged over all five survey periods, was used because we assumed that roe deer use of the plots would not significantly change between seasons (Appendix S1).

| Statistical analysis
For each woody-plant species, we assessed the impact of surrounding understory characteristics on browsing intensity. We calculated species-specific models for woody-plant species with data from For each woody-plant species, we fitted separate generalised linear mixed models (glmm) for each individual plant, with browsing intensity index as the response. We assumed a binomial data distribution as we used proportional data derived from counts (Douma & Weedon, 2019). All continuous variables were scaled. In every model, we used the same set of explanatory variables ( Table 1). We used the height of each individual plant with a quadratic effect, to TA B L E 1 account for a potential optimal browsing height. To assess the potential effect of deadwood on roe deer browsing intensity by obstructing deer movement we added the variable Deadwood describing the volume of lying deadwood on the scale of the research plots (Table 1). Finally, we included a nested random effect for transect within research plot to account for spatial dependence of woody plants within the transect and the plot.
Following data inspection according to Zuur et al. (2010), we excluded total woody plants from the models for beech and spruce, due to collinearity with a number of conspecifics (Dormann et al., 2013). The variable Grouping was dropped from the speciesspecific model for F. excelsior as no individual in close groups were surveyed, thereby resulting in only one category. We selected all best-fitting candidate models with an ∆AICc < 2 and report the resulting conditional averaged models. Results were deemed to be significant when alpha was 0.05 or smaller. Model assumptions were visually assessed.
To compare the browsing intensity among species without the possible effects of other variables, we fitted another glmm, using species as a variable and influential predictors from the speciesspecific models as random effects (Height, UnderC, UnderDiv, and Year). To assess the difference in browsing intensity between summer and the full year we used the Wilcoxon ranked-sum test to compare the browsing intensity from the total autumn and spring survey. Surveys of autumn and spring were not truly independent, and the unequal sample size did not allow us to use a paired test.
We performed all analyses in R 4.1.2 (R Core Team, 2021). For the species-specific glmms, we used the glmmTMB function (Brooks et al., 2017), while model selection was conducted using MuMIn (Barton, 2020).

| RE SULTS
We sampled a total of 11,123 and 10,576 individual woody plants from autumn and spring surveys respectively, belonging to seven species and one additional genus ( Note: We present the sample size per species (N), uncorrected mean and standard deviation of branches browsed (mean (SD)), and proportion of branches browsed on each individual plant (proportion branches browsed). Autumn survey reflects browsing from summer of the same year, while the spring survey assessed browsing in the previous summer and spring. Percent of browsing in winter is the change in browsing between subsequent surveys, stated as a percentage increase. Wilcoxon test gives the results comparing the browsing intensity between autumn and spring.

| Drivers of browsing pressure
The results of the species-specific models represent the conditional averaged model results assessing the effects of understory vegetation, roe deer abundance and deadwood on roe deer browsing pressure on woody plants (

| DISCUSS ION
The influence of understory characteristics through associational and conspecific effects on browsing intensity by roe deer varied between woody plant species. We found associational and conspecific effects for species that were browsed with intermediate intensity by roe deer (Question 1 and 2). F. excelsior was browsed less as overall understory cover increased, while browsing of V. myrtillus increased with understory cover, but decreased with the abundance of wood plants and number of conspecifics.
Browsing of A. Alba decreased with understory cover and proximity to conspecifics and increased with understory cover and diversity. Browsing of V. myrtillus, and A. Alba increased with relative roe deer abundance, but roe deer abundance had a negative impact on browsing of A pseudoplatanus (Question 3). Finally, lying deadwood had no effect on the browsing pressure of any plant species (Question 4). preference for roe deer. Individuals of these species are either always browsed upon, or always avoided by roe deer independent of the characteristics of the understory.
High Shannon-diversity of the understory increased the proportion of A. alba branches browsed indicating associational susceptibility but had no effect on other woody plant taxa. We assume that increased plant diversity also increases the availability and diversity of attractive forage plants for roe deer Ohse et al., 2017). Roe deer, as concentrate selectors, prefer to browse on a variety of food plants, and a higher patch diversity may increase the attractivity of the site which leads to a high site use by roe deer. Consequently, proximity to attractive resources increases the opportunistic use of a less attractive resource and thus leads to associational susceptibility (Bergvall et al., 2006;Courant & Fortin, 2010;Häsler & Senn, 2012). Other studies found contrasting effects of plant diversity on browsing intensity of individual focal plants. Similar to our study, Vehviläinen and Koricheva (2006) and Milligan and Koricheva (2013) found an increase of browsing pressure with higher patch diversity, while Champagne, Dumont, et al. (2018) and Ohse et al. (2017) found reduced browsing on sites with high Shannon diversity or species richness respectively. Overall, the effect of understory diversity depends likely on the species identity of the occurring plants and their respective quality as forage plants, but probably also their availability on larger spatial scales. In general, the abundance of high-quality forage plants will generally increase attraction of herbivores to the site and thus increase herbivory (Bee et al., 2009;Champagne et al., 2020;Ohse et al., 2017;Wang et al., 2010). Due to differences in feeding preferences, food-niche width and selectivity, the associational effects resulting from high or low forage quality are herbivore-specific (Barbosa et al., 2009;Champagne, Dumont, et al., 2018;Vehviläinen & Koricheva, 2006).
Woody plant abundance had contrasting effects on roe deer browsing intensity, namely more woody plants increased browsing pressure on A. alba and decreased browsing pressure on V. myrtillus, which might be due to multiple effects. High abundances of woody plants specifically may provide increased hiding cover as well as protection from environmental conditions in addition to forage and may increase site selection for roe deer (Bobrowski et al., 2020;Gill et al., 1996;Ohse et al., 2017), consequently leading to browsing susceptibility of other plants. However, woody plants can also physically protect neighbouring trees from browsing, thus functioning as nursing plants (Ameztegui & Coll, 2015;Gómez-Aparicio et al., 2008;Smit et al., 2007). In this case, the contrasting directionalities might be due to species identity or physical structure of the woody vegetation, for example, reduced browsing on V. myrtillus could be due to higher preference of roe deer for other woody plants. For comparison, Champagne, Dumont, et al. (2018) found that a higher number of available shoots increased browsing pressure.
Although habitats with high understory cover are attractive for roe deer (Heinze et al., 2011;Tufto et al., 1996), the understory cover can deflect browsing from individual A. alba and F. excelsior.
Higher forage availability may lead to reduced selection of specific plants for browsing, however, the current literature does not always support this idea. For example, Kupferschmid et al. (2020) found an increase in browsing on seedlings with higher understory cover, while other studies found a decrease in browsing with higher understory cover (Verheyden-Tixier et al., 1998), or no effect (Bergquist & Örlander, 1998). The discrepancies are likely influenced by confounding effects, such as herbivore species studied, its feeding behaviour (i.e. browser or grazer) and the composition large herbivore community. Less selective herbivores than roe deer (e.g. red deer, Gebert & Verheyden-Tixier, 2001) might for example, select vegetation patches due to the available volume of forage and less for its quality, leading associational susceptibility for individual woody plants. In our study, V. myrtillus was browsed more when understory cover increased. V. myrtillus is not of high preference for roe deer, but is still an important food source due to its general high availability and its tendency to outcompete other plants in the understory Petersson et al., 2019;Tixier & Duncan, 1996). Thus, sites with high overall understory cover, with other more attractive resources may lead to associational susceptibility for V. myrtillus.

| Conspecific effects
Overall, we found conspecific effects for A. alba and V. myrtillus. For A. alba grouping with conspecifics reduced browsing impact, showing conspecific resistance, likely due to a dilution effect (Champagne et al., 2020

| Relevance of roe deer abundance
Abundance of herbivores is often thought to drive browsing pressure on plants Klopcic et al., 2010;Ward & Williams, 2020). Contrary to our expectations, we found that this is only true for A. alba and V. myrtillus, while roe deer abundance did not increase the browsing of any other plant species (similar to Kamler et al., 2010;Wright et al., 2012). While culling of deer can help reduce browsing pressure (Hothorn & Müller, 2010;Ward & Williams, 2020), it may only help certain woody-plant species, specifically those that are not of high interest to deer (Kamler et al., 2010). Additionally, our results show that using browsing indices as a proxy for ungulate density (e.g. in management) may only be an imprecise measure, weakly related to actual ungulate abundance.

| The impact of lying deadwood
We did not find evidence that lying deadwood affected roe deer browsing intensity on woody plants. Lying deadwood has been suggested to pose a physical barrier for deer and thus locally reduces browsing on woody plants , however, the results in the literature are not consistent (e.g. Kupferschmid et al., 2020;Pellerin et al., 2010). In our own study system, we previously demonstrated localised site avoidance by roe deer in autumn where deadwood was abundant , but in the present study, we cannot demonstrate reduced browsing intensity with increasing deadwood volume. The volumes of deadwood on our study sites (on average 36.24 m 3 /ha, total range 0-297 m 3 /ha) are significantly lower than reported from natural or primaeval montane beech-fir forests (on average 223.9 m 3 /ha) (Bujoczek et al., 2018).
The relatively low amounts of deadwood on our research sites might mediate roe deer habitat use, but not physically inhibit browsing once deer are present. Thus, the potential lower browsing impact would already be explained by relative roe deer abundance in this study. Future studies should investigate the relationship between deadwood and browsing in more detail considering multiple spatial scales as well as a wider gradient in lying deadwood volume. Forest managers possibly need to retain larger volumes of deadwood, if it should be used to reach management goals towards natural tree succession.

| Non-uniformity in browsing susceptibility and intensity
Understory characteristics can have a significant influence of the future composition of forests, by moderating deer browsing patterns and shaping potential filtering effects among tree species (Begley-Miller et al., 2014;Chollet et al., 2021). Silver fir is a highly relevant species for forestry (Schwarz & Bauhus, 2019)

| Limitations and next steps
Like the literature, we did not find definitive effects of the studied variables on browsing intensity (Champagne, Dumont, et al., 2018;Kupferschmid et al., 2020;Milligan & Koricheva, 2013;Verheyden-Tixier et al., 1998). This may be because these metrics alone are not yet sufficient to understand associational effects and their directionality. For example, in our study, plant species diversity increased the likelihood of individual fir trees being browsed. However, this might also depend on the overall plant diversity in the landscape and the opportunities roe deer have for resource patch selection, which in turn is affected by forest management (Hedwall et al., 2019;Márialigeti et al., 2016). In our study area, management for timber production excludes late successional forest stages which are typically rich in plant species (Scherzinger, 1996), likely leading to low overall understory diversity. We speculate that in a forest with higher large-scale plant diversity, individual patches of diverse vegetation might not stand out as very attractive for roe deer, as many other similarly attractive sites are available, which would reduce the extent of associational susceptibility we found in our results.
Consequently, larger scale patterns as well as local site conditions, for which forest management can be a strong driver, likely affect the extend and directionality of associational effects.
In our study area, high human disturbance in the forest, lack of natural predators, and intense hunting to control roe deer densities may have also influenced our result. Roe deer may trade-offs between risk and resource selection in areas with high human disturbance (e.g. hunting, recreation; Bonnot et al., 2013;Gerhardt et al., 2013;Möst et al., 2015), and thus the relationship between roe deer abundance and browsing may be different in areas with lower human presence. Finally, associational effects also depend on the population density of the herbivore species (Vehviläinen & Koricheva, 2006

| CON CLUS ION
In our study, we demonstrate that browsing on species that are of very low or high preference to roe deer is not affected by the characteristics of the understory. However, our study shows that understory characteristics are a strong driver for some woody-plant species like for example A. alba. Specifically, our results show that woody plants can be exposed to association resistance, susceptibility and conspecific effects at the same time by different characteristics of the understory. Thus, forest managers should take understory characteristics into consideration when trying to counter browsing damage by roe deer. Increasing overall forage availability, possibly by opening the canopy, could for example contribute to reduce overall browsing damage and increase species richness in the canopy and consequently forest resilience long-term. Unlike what one would expect, roe deer abundance was only related to browsing on some woody species like silver fir. Managers should be aware that browsing pressure is not solely dependent on deer abundance, thus culling alone, might not be an effective measure to reduce browsing pressure on all woody-plant species. Instead, managers should aim to integrate management of food resources (e.g. by increasing overall understory cover) into their toolbox in order to decrease browsing pressure on young trees.

ACK N OWLED G M ENTS
We thank Jan Helbach for his data. We are most grateful to Forst Baden-Württemberg (ForstBW) for enabling and facilitating field work -without their excellent work, our study would not have been possible. We gratefully acknowledge support by the German Science Foundation (DFG), Research Training Group ConFoBi (GRK 2123/2).
We thank Dr. Champagne and two anonymous reviewers for helpful comments and suggestions. Open Access funding enabled and organized by Projekt DEAL.

FU N D I N G I N FO R M ATI O N
This study was funded by the German Research Foundation (DFG), ConFoBi project no. GRK 2123.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare no conflict of interests.